Building Wireless Sensor Networks

Theoretical and Practical Perspectives

Nandini Mukherjee, Sarmistha Neogy, Sarbani Roy

• Publisher: CRC Press
• Year: 2017
• ISBN:
  9781351831192
• Edition: 1
• Topic: Computer Science
• Subtopic: Computer Networking
• Index: Computer Science

Building Wireless Sensor Networks: Theoretical and Practical Perspectives presents the state of the art of wireless sensor networks (WSNs) from fundamental concepts to cutting-edge technologies.

Focusing on WSN topics ideal for undergraduate and postgraduate curricula, this book:

• Provides essential knowledge of the contemporary theory and practice of wireless sensor networking
• Describes WSN architectures, protocols, and operating systems
• Details the routing and data aggregation algorithms
• Addresses WSN security and energy efficiency
• Includes sample programs for experimentation

The book offers overarching coverage of this exciting field, filling a critical gap in the existing literature.

1
Introduction

Sensors have been in use for a very long time in traditional applications. These applications include touch-sensitive sensors on buttons of microwaves and elevators, motion detector sensors which turn lights on and off, smoke detector sensors, etc. With advances in electronics and communication technology sensors are now being used in many new applications which were not even considered only a decade back. Particularly, due to rapid development of wireless networks, it has been possible to deploy a network of sensors spread over a large geographical area to allow sensor devices communicating wire-lessly to gather huge volume of data for use in several new applications which had not been envisioned earlier.

Our intent is to present to the readers the advancements in wireless sensor networks (WSNs), both in theoretical and practical perspectives and also enable them to write small applications in a WSN environment. Thus, with an overview of sensor and mote technologies and a discussion on issues and challenges of building WSNs in this chapter, the book will gradually focus on wireless communication protocol standards, routing and data aggreagation algorithms, localization techniques and algorithms, energy conservation and security issues. The last two chapters of the book concentrate on providing a practical guide to the readers for programming the sensor motes to develop small applications.

1.1 Sensors

Sensors are used to sense a wide range of parameters represented by different energy forms such as movement, electrical signals, thermal or magnetic energy, etc. Sensors are able to sense a physical change in some physical characteristic which changes in response to some excitation, for example, heat. The change in such a characteristic is converted to an electrical signal.

Any sensor produces a voltage (or signal) which is proportional to the change in the parameter measured. The type and amplitude of the output signal depend on the sensor type.

Active and Passive Sensors

Broadly, a sensor is classified as a passive sensor or an active sensor. Active sensors require an external power supply to operate. This external source of energy is called an excitation signal. Active sensors measure changes of their own properties in response to the external effect.

An example of an active sensor is a strain gauge which is used to measure strain on an object. Its electrical resistance can be measured by detecting variations in the current (or voltage) across it and relating these changes to the amount of strain or force applied, but this measurement requires a current to be passed through the gauge.

On the other hand, passive sensors do not require any additional energy sources. They are direct sensors which change their physical properties such as resistance and capacitance and generate electrical signals in response to an external stimulus. An example of a passive sensor is a photo-diode. When an external stimulus like light excites a photodiode, photons are abosrbed in the photodiode and current is generated.

Analog and Digital Sensors

Analog sensors produce continuous signal which is proportional to the parameter measured. Many physical parameters such as temperature, pressure, and displacement are analog quantities and they are measured as continuous signals.

Digital sensors produce discrete signals which are digital representations of the quantities of the parameters being measured. These discrete vaues are output as a single bit or a group of bits representing a quantity.

Properties of Sensors

A good sensor must obey the following rules:

• It should be sensitive to the measured property.
• It should be insensitive to any other property.
• It should not influence the measured property.

In an ideal situation, the output signal of a sensor is exactly proportional to the value of the measured parameter. The gain is then defined as the ratio between output signal and measured parameter. For example, if a sensor measures temperature and has a voltage output, the gain [V/K] (V is voltage and K is temperature) is a constant with the unit.

An important consideration for a sensor is its area of coverage defined as the geographical region in the proximity of a sensor which is covered by it. A sensor can measure every change in the physical properties within that region.

Table 1.1 Commonly used sensors

Quantity Measured	Sensor
Light Level	Light Dependent Resistor (LDR)
	Photodiode
	Phototransistor
	Solar Cell
Temperature	Thermocouple
	Thermistor
	Thermostat
	Resistive Temperature Detector
Force/Pressure	Strain Gauge
	Pressure Switch
	Load Cell
Position	Potentiometer
	Encoder
	Reflective/Slotted Opto-switch
	LVDT
Speed	Tacho Generator
	Reflective/Slotted Optocoupler
	Doppler Effect Sensor
Sound	Carbon Microphone
	Piezoelectric Crystal

(Source: http://www.electronics-tutorials.ws)

1.2 Sensor Node Architecture

A sensor node, also known as a mote, is a building block in a wireless sensor network. In addition to sensing capabilities, a sensor node also wirelessly communicates with other nodes in the network to propagate information through the network. As we will see in the subsequent chapters, a sensor node must also be capable of performing some processing tasks. Thus, the major functionalities that a sensor node must perfom include sensing data from the environment, such as temperature, or motion, processing the data as required by the application and communicating with other nodes in the network. Figure 1.1 depicts the architecture of a sensor node.

Figure 1.1 Sensor node architecture

As depicted in the figure, a sensor node consists of a microcontroller, some amount of memory, a tranceiver and one or more sensors embedded in it. Sensor nodes are generally deployed in places where no external power sources are available. Therefore, sensor nodes are battery-powered and a power supply is integrated with it.

Sensing Subsystem

The sensing subsystem in a sensor node includes one or more sensors. For example, a node may be capable of sensing three parameters from the environment such as temperature, humidity and light if these three sensors are embedded in it. In the case of analog sensors, an analog-to-digital converter (ADC) is used to convert the analog output signal of a sensor into a digital signal.

Processing Subsystem

The processor subsystem interconnects all the other subsystems and some additional peripherals. Its main purpose is to execute instructions pertaining to sensing, communication and self-organization. This subsystem consists of a processor chip, a nonvolatile memory which stores program instructions, an active memory which temporarily stores the sensed data and sometimes processed data and an internal clock.

As a processing element, a mote (sensor node) often uses a microcontroller. A microcontroller contains a CPU core, a volatile memory (RAM) for data storage, a ROM, EPROM, EEPROM or flash memory, parallel I/O interfaces, discrete input and output bits, a clock generator, one or more internal analog-to-digital converters and serial communications interfaces.

Microcontrollers are of small size, low cost and their power consumption is low. Hence they are suitable for building computationally less intensive applications. However, microcontrollers are less powerful and less efficient in comparison with custom-made processors. The other options are digital signal processors (DSPs), application-specific integrated circuits (ASICs) and field programmable gate arrays (FPGAs).

DSPs process discrete signals with simple electronic circuits like adders, multipliers and delay circuits. Digital filters are used for reducing the noise effect and enhancing or modifying spectral characteristics. DSPs usually are based on Harvard architecture and are powerful and efficient. They can be used for applications where nodes are deployed in harsh physical settings and signal transmission may be affected by noise. However, DSPs are not suitable for tasks requiring periodic upgradation and modification.

An ASIC is actually an integrated circuit which can be customized for a specific application. Sometimes, a half-customized ASIC is built with logic cells that are available in the standard library. Whether an ASIC is fully customized or half-customized, the final logic structure is configured by the user. An ASIC can be optimized to meet the requirements of an application. However, its development cost is high and re-configuration is difficult. ASICs are used not to replace microcontrollers or DSPs but to complement them.

In comparison with ASICs, FPGAs are more complex in design and more flexible to program. FPGAs are programmed by modifying a packaged part. Programming is done with the support of circuit diagrams and hardware description languages, such as VHDL and Verilog. Although, FPGAs are complex and undergo an expensive design and realization process, there are some advantages of using them. FPGAs have higher bandwidth compared to DSPs, support parallel processing, can work with floating point representations and provide greater flexibility of control.

Communication Subsystem

In a wireless sensor network, fast and energy-efficient data transfer between the sensor nodes is important. However, the sensor node sizes are made small, so that they can be deployed on a large scale over a large geographical area. The size of sensor nodes puts restrictions on system buses and parallel transmission cannot be supported. Usually a high speed, full duplex, synchronous serial bus is ...

Table of contents

• Cover
• Half Title
• Title
• Copyright
• Dedication
• Contents
• List of Figures
• List of Tables
• Preface
• Acknowledgments
• Authors
• 1: Introduction
• 2: Wireless Sensor Networks Architecture
• 3: Information Gathering
• 4: Energy Management in WSN
• 5: Security in WSN
• 6: Operating Systems for WSNs
• 7: Programming WSNs
• Index